Leaks can form in HVAC/R systems for a variety of reasons including corrosion and physical damage. The loss of refrigerant from a leak causes system components to freeze and will eventually cause the system to stop functioning. Replacement of the lost refrigerant is costly and emissions of refrigerant vapors are damaging to the environment.
Several leak sealant formulations have been developed over the years for use in HVAC/R systems. Many of these are based on the polymerization of organosilanes as taught by U.S. Pat. No. 4,237,172. Alkoxysilanes have hydrolysable alkoxy groups that react with water to form an alcohol and a silanol. The silanols rapidly condense with each another to form an insoluble polymeric network of organosilanes. These reactions are meant to occur at the leak site where atmospheric moisture condenses due to the presence of a local temperature minimum caused by evaporating refrigerant. It is detrimental to have the leak sealants polymerize in the interior of the system away from a leak site because the chemicals are consumed before leaks can be sealed, and polymerization can clog internal HVAC/R components or negatively affect the lubrication qualities of the oil.
Moisture can enter the refrigeration system through improper evacuation procedures or by the service technician or manufacturer inadvertently introducing oil that is saturated with moisture due to extended exposure of the oil to the ambient air (which usually contains very high levels of moisture relative to the limits in a refrigeration system). This undesirable internal system moisture can react with sealants. It is therefore critical to ensure that the system is largely free of moisture before introducing the sealant.
Despite all attempts to keep the HVAC/R system as dry as possible, moisture can still enter the system and be absorbed into the oil. Although polyolester (POE) oil is hygroscopic and will readily absorb moisture upon exposure, even mineral oils can absorb moisture albeit their saturated moisture levels being much lower about 25 parts per million (ppm) for mineral oil compared to 2,500 for POE oil, 6,500 for polyvinylether (PVE) oil and 10,000 PPM for polyalkylene glycol (PAG) oil. Even with mineral oil's much lower saturation moisture levels, however, it is typically not practical to change out the oil and the refrigerant in the event of moisture contamination.
One way to remove moisture from an HVAC/R system to prevent side-reactions with leak sealants is by the introduction of a chemical additive to the system that consumes water via a chemical reaction. U.S. Patent Application No. 2014/0165625 A1 teaches the use of hydrolytic orthoesters drying agents which are introduced into an operating HVAC/R system. Orthoesters are specifically emphasized as hydrolytic moisture scavenging additives and only broadly suggests the use of hydrolytic drying agents (e.g. acetals, epoxides and carbodiimides). While orthoesters, specifically orthoformates, remove water from refrigeration oils, they can react to form an ester and an alcohol, which in and of itself would be acceptable; however, esters can then be hydrolyzed further to form acid (e.g. formic acid) under certain conditions which is highly undesirable in HVAC/R system. Therefore, an improved moisture removal additive is desperately needed for HVAC/R systems that would not potentially hydrolyze to form acids since acids are responsible for corrosion, compressor burn-outs and continued oil/refrigerant degradation.
It is also well known in the art to use silanes as moisture scavengers; however, the use of silanes can interfere with the action of silane-based leak sealing additives and therefore is not optimal. That is, the silanes used for moisture removal are selected to form soluble polymers that stay dissolved in the oil whereas silanes used to seal leaks are designed to form highly cross-linked insoluble polymers at the leak site. The moisture removing silanes could thus prevent cross-linking of the leak sealing chemicals and reduce their effectiveness.
To resolve the problem of undesired by-products of the moisture drying reaction, we discovered a drying formulation for removing moisture from the refrigerant and oil in operating HVAC/R systems. The drying agent capitalizes on the hydrolysis of 2,2-dimethoxypropane to remove the moisture from the refrigerant and oil by consuming the water molecules and advantageously producing methanol as well as producing acetone which does react any further in the HVAC/R system. The problem that occurs when using orthoformates, namely the buildup of acid due to over-hydrolysis of orthoformates that are used in HVAC/R drying formulas such as the above-referenced U.S. Patent Application No. US2014/0165625 A1 is avoided by the use of our invention.
As stated earlier, while U.S. Patent Application No. 2014/0165625 A1 only broadly suggests the use of hydrolytic drying agents including acetals and 2,2-dimethoxypropane is also known as acetone dimethyl acetal, the prior art did not recognize, as we did, that 2,2-dimethoxypropane is a far superior drying agent that will not form unwanted harmful reaction products. Furthermore, the prior art did not realize that this compound's reaction with the water molecules produces methanol which will liberate the trapped acid from the oil allowing it to vaporize and travel to the filter drier, thereby accelerating the removal of acid from the system. United States Air Force Contract AF 33(616)-7006 taught the use of 2,2-dimethoxypropane, anhydrous hydrazine, trimethyl orthoacetate, trimethyl orthoformate, trimethyl orthovalerate as well as two silanes only to prevent fuel filter icing in mineral-oil based aviation fuels but provided no guidance for use of 2,2-dimethoxypropane in HVAC/R systems or even hinted at their superiority in not further hydrolyzing to acid, as is possible with orthoesters.
U.S. Patent Application No. 2014/0165625 A1 describes experiments with orthoester drying agents in mineral oil doped with 1 wt % water. This scenario is an extreme case given that mineral oil will only absorb approximately 25 ppm moisture. Therefore, the 1 wt % water will be largely present in a second phase separate from the mineral oil. This is not reflective of typical conditions inside an HVAC/R system when contaminated with moisture. U.S. Patent Application No. 2014/0165625 goes on to teach that the reaction rate of moisture with the orthoester drying agent is faster than the reactions of moisture with leak sealants. Faster reaction rate is said to allow both the drying agent and leak sealant to be injected into the system at the same time because the drying agent will react with any moisture present before detrimental polymerization of the leak sealant can occur inside the HVAC/R system. However, our experiments, with moisture levels that are both realistic and characteristic of HVAC/R systems, have shown this not to be the case. Even for POE lubricants which have a much larger capacity for moisture absorption (2,500 ppm verses 25 ppm for mineral oil), the rate of reaction with the drying agent is normally not sufficient to prevent polymerization of the leak sealant if they are introduced simultaneously. This is because the rate of hydrolysis of the drying agent is much slower in oil solvent, as compared to in a separate aqueous phase as was the case in Table 6 of U.S. Patent Application No. 2014/0165625.